The present invention relates to typewriters, and more particularly to a changeable type element which may be removably mounted on a typebar.
Known prior art changeable type elements for typebars have incorporated locking devices to prevent the type element from becoming disengaged from a typebar and from being propelled from the typebar while in flight.
For example, one known changeable type element, shown in U.S. Pat. No. 1,090,597 to S. D. Busz, incorporated a hollow open ended type element with type characters embossed on opposite faces that connected the walls. The type element was placed upon a preformed portion of the typebar and locked in position by a pivotal arm. The pivotal arm included an overlying detent portion that engaged a recess in the top of the type element. The centrifugal force of the typebar during flight forced the type element against the detent portion of the pivotal arm and was prevented from disengaging. If the detent portion of the pivotal arm became deformed, the detent portion could disengage from the type element. If this happended and the typebar was in flight, the type element would dislodge from the typebar and fly off.
Other changeable type devices, such as that shown in U.S. Pat. No. 3,308,916 to F. H. Canny et al., included a U-shaped changeable type element having a single font bearing surface. A projecting abutment on its rear surface engaged a recess in the typebar as the type element was assembled on the typebar. A spring attached to the type element biases the type element into engagement with the typebar. Excessive shock during impact of the type element against the platen may overcome the biasing effect of the spring and the type element may dislodge from the typebar and fly off.
None of the prior art included means to compensate for wear on the engageable parts as the present invention does.